The present invention relates to a reference mark selection system for a measuring apparatus of the type which comprises a measuring scale, a measuring graduation defined on the scale, a plurality of reference marks positioned at predetermined positions on the scale with respect to the graduation, and a scanning unit which includes means for generating a reference signal when the scanning unit is aligned with one of the reference marks.
In such a measuring apparatus, the reference signals can be used to generate control pulses which may be used in various ways. For example, such control pulses can be used to set a display counter to a zero position, to load a predetermined constant value into the counter at the commencement of a measurement operation, to control interference pulses, as well as for controlling devices coupled to the measuring system.
German DE-AS No. 25 40 412 discloses an incremental measuring system in which a measuring scale is provided which defines a measuring graduation. A series of reference marks are formed on the scale at a fixed spacing with respect to one another, and at least one selection element is arranged on the scale itself or in the immediate vicinity of the scale such that the selection element is slidable in a groove. The selection elements operate to select one or more reference marks which are intended to go into operation during the measuring process. The selection elements are detected by a switching means included in the scanning unit. This switching means is mounted to detect the magnetic selection elements and to generate a selection signal when the scanning unit is in alignment with one of the selection elements. This selection signal is applied as an input to a logic circuit, along with the reference signals generated by the scanning unit. This logic circuit generates a control pulse at its output only if the selection signal and the reference signal are simultaneously present on inputs of the logic circuit.
German Pat. No. 30 37 810 describes a similar incremental measuring system with reference marks provided along the graduation of the scale. In the disclosed system, appropriate ones of the reference marks are selected by means of an optoelectronic device. The selection element, which may for example take the form of a light source, is slidable in a groove of the scale carrier. The selection element can be moved into alignment with the selected reference mark. The scanning unit includes a photosensor arranged so as to detect the selection element when the scanning unit moves into alignment with the selection element. This photosensor generates a selection signal similar to that described above which enables the associated reference signal to go into operation. The positioning of the selection element in the groove in order to align the selection element with a desired reference mark may also take place by means of followers included on the scanning unit.
Such incremental measuring systems are used, for example, in processing machines to measure the relative position of two machine parts. Such machines typically generate vibration and jarrings, which can cause the selection elements to be displaced. If this happens, a clear alignment of the selection element with the selected reference mark and therefore activation of the reference mark can no longer be assured. In the event two or more reference marks are closely adjacent to one another, such displacements of the selection element may even cause the wrong reference mark to go into operation so that an erroneous control operation, for example a zeroing operation, is triggered in the counter. Such errors will typically result in erroneous measuring results. For this reason, it is important that each selection element which is shiftable in the direction of movement of the scanning unit must be fixable in place with respect to the desired reference mark. Furthermore, it is important that each selection element be fixable in a reproduceable position. Another problem encountered by the prior art is that when selection elements take the form of magnets, such magnets attract metal shavings to a strong degree which are difficult to remove. Similarly, the removal of contaminants and impurities from optical selection elements (which may take the form of light sources, mirrors or prisms) is not always readily achievable.